1. Field of the Invention
The present invention relates to the adjustment of the lightening of a working unit of a machine, which working unit is carried by a carrying structure in such a way as to rest on the ground during work and be held away from the said ground when work is interrupted or during transport, it being possible for the said unit to be lowered so as to be brought to rest on the ground or to be raised so as to be brought away from the said ground by means of at least one hydraulic-energy converter fed by an oil-supply source through a hydraulic circuit.
By way of entirely non-limiting example, such machines may be encountered in the field of agriculture, civil engineering works, the upkeep of roads and verges, etc.
2. Discussion of the Background
The amount by which the working unit of such a machine is lightened is generally of great importance as regards the quality of the work done. Excessive lightening does not allow the working unit to remain continuously in contact with the ground. This will lead to uneven work. Insufficient lightening, on the other hand, in some cases increases the risk of clogging, increases the power that needs to be developed to move the working unit, and increases wear.
In the field of mowers, for example, it is known practice for the harvesting mechanism (working unit) to be lightened by means of a lightening device comprising at least one elastically deformable member, the action of which can be adjusted by means of an adjusting device coupled to the said elastically deformable member.
As elastically deformable members, it is known practice to use mechanical tension springs (GB 2 053 645 A), mechanical compression springs, torsion bars, deformable cushions filled with air (EP 0 570 315 A1) or nitrogen-filled accumulators combined with hydraulic rams (EP 0 149 870 A1).
The lightening provided by these elastically deformable members is generally adjusted manually and requires a number of tests, with work being interrupted, and this may lead to time being lost especially since, as the initial adjustment is carried out when the working unit is clean, there is the risk that it will soon become inappropriate during work as a result of a build-up of earth, for example.
Various solutions have already been proposed to facilitate this adjustment.
Described in document HP 0 149 870 A1 is a solution which consists in constantly measuring the pressure with which the working unit of a machine rests on the ground. The working unit is provided with a runner which is connected to it so that it can pivot. A piston and cylinder sensor is fitted between the said runner and the chassis of the working unit. A conversion device converts the pressure in the piston and cylinder sensor into an electrical signal which controls an electric motor. This electric motor drives a pinion which meshes with a rack actuating the rod of a compensation ram equipped with gas accumulators. This compensation ram is in communication with hydraulic rams fitted between the chassis of the working unit and the arms of the wheels via which the working unit runs along the ground.
When the pressure in the piston and cylinder sensor increases following an increase in weight of the working unit, the electrical signal created actuates the electric motor in a direction such that the pressure in the compensating ram and the hydraulic rams increases. This correction takes place entirely automatically until the correct pressure is achieved.
Described in document WO 86/00002 is a solution which consists also in measuring the pressure with which the cutting head (working unit) of a harvesting machine rests on the ground. The cutting head of this harvesting machine is carried by a chassis coupled to a tractor and comprising two wheels by means of which it runs along the ground. Each wheel is connected in a pivoting fashion to a corresponding side wall of the chassis by means of a wheel arm. A hydraulic ram associated with a hydropneumatic accumulator is fitted between each wheel arm and the corresponding side wall.
The cutting head comprises a certain number of runners which are connected in pivoting fashion to the bar of the said cutting head. A strain gauge is fixed on the top of a runner and supplies an electrical signal which is a function of the change in pressure exerted by the said runner on the ground. This electrical signal is transmitted to an amplifier with two outputs, each of which is connected to an electrovalve. These electrovalves allow the hydraulic rams to be connected to an oil-supply source. Depending on the signal emitted by the strain gauge, the amplifier controls the appropriate electrovalve with a view to feeding the hydraulic rams with oil, or to respectively withdrawing oil from the said hydraulic rams in order to raise, or respectively to lower, the cutting head until the desired pressure with which the cutting head should rest on the ground is re-established.
These two solutions do, however, have a certain number of drawbacks.
A first drawback lies in the fact that the sensors (piston and cylinder in EP 0 149 870 A1 and strain gauge in WO 86/00002) have to be placed under the working unit which, in some types of machine, may prove prejudicial to the quality of work done.
A second drawback lies in the fact that these sensors and the connections which transmit the information they supply, have to be situated in places where, bearing in mind the environment in which these machines work, there is a risk that they will soon become damaged.
A third drawback lies in the fact that the number of parameters is such that correct lightening is rarely achieved. By way of non-limiting example, mention may be made of the nature of the ground, for example: in firm ground, the runners will effectively slide over the surface of the ground, whereas in soft ground, they will dig into the ground a little bit and this will change the information supplied by the sensors. Now, the user is not generally in a position to supply the new values of the parameters to the device which manages the information received from the sensors.
Finally, a fourth drawback lies in the fact that the device constantly corrects during work, and this may also have a harmful influence on the quality of work done.
Described in document EP 0 741 960 A1 is a solution in which the user can adjust the lightening of the harvesting mechanism (working unit) of a mower from the driving position of the motor vehicle to which the mower is connected.
In a first embodiment (FIGS. 6 and 9) the lightening device comprises two elastically deformable members, the action of each of which can be adjusted by means of an adjusting device coupled to the said elastically deformable members each of which consists of a deformable envelope containing gas at an adjustable pressure (air cushion in FIG. 6, and hydropneumatic ram in FIG. 9).
This adjustment device in particular comprises a feed circuit equipped with one valve per deformable envelope intended, with a view to adjusting the value of the pressure of the gas contained in the said deformable envelope, to let into the deformable envelope a fluid delivered by a feed source (air delivered by a compressor in FIG. 6 and oil delivered by the hydraulic system of the motor vehicle in FIG. 9), to maintain the pressure of the gas contained in the deformable envelope, or to let fluid out of the said deformable envelope. For this, each valve is an electrovalve that can be controlled electrically by the user from the driving position of the motor vehicle and having two positions: a first position in which fluid can neither get into nor out of the deformable envelope and a second position in which fluid can get into or out of the deformable envelope.
In a second embodiment (FIG. 12), the lightening device comprises two mechanical springs, the action of which can be adjusted by means of an adjusting device which comprises a ram for each mechanical spring and a circuit for feeding these two rams. This feed circuit is equipped with a valve associated with each ram and intended, with a view to adjusting the action of the corresponding mechanical spring, to let fluid delivered by a feed source into the ram, to maintain the fluid in the said ram or to let fluid out of the said ram. For this, each valve can be controlled by the user from the driving position of the motor vehicle and has two positions: a first position in which the fluid can neither get into nor out of the ram, and a second position in which fluid can get into or out of the ram.
In these known mowers, the user can, from the driving position of the motor vehicle, either increase or decrease the action of the two elastically deformable members, as the case may be, until they work in the optimum way. Moreover, a skilled driver can carry out this operation without interrupting the mowing.
If, during work, the user notices that the behaviour of the mower has changed, all he will need to do, without leaving his driving position, or even, as already stated, without interrupting the mowing, will be to readjust the value of the action afforded by the elastically deformable members.
It is true that in these known mowers the value of the lightening is simple and quick to adjust, but it does require the intervention of the user.
Described in document FR 2 701 801 A1 is a device for automatically adjusting the lightening of a working unit of a machine. This working unit is carried by a carrying structure so that it rests on the ground during work and is held away from the said ground when work has to be interrupted or during transport. This working unit may be lowered to be brought to rest on the ground or raised to be moved away from the said ground by means of the said device which comprises hydraulic-energy converters fed by a hydraulic circuit comprising in particular a hydropneumatic accumulator. The pressure of the gas contained in this hydropneumatic accumulator can be set initially to a value that corresponds to the desired amount of lightening of the working unit.
This hydraulic circuit comprises a main circuit which allows the working unit to be raised or lowered, and a secondary circuit which affords predetermined lightening of the working unit once it is resting on the ground.
Furthermore, this secondary circuit makes it possible to maintain this predetermined lightening irrespective of the variations in level of the ground on which the working unit is resting.
By contrast, if, following the build-up of earth on the working unit, the weight of the latter has increased, the user has manually to modify the amount of lightening if he wishes to return to the same apparent weight of the working unit.